v6.db.transport.rest is a REST API for the public transportation system in Germany.
Because it wraps an API of Deutsche Bahn, it includes most of the long-distance and regional traffic, as well as some international trains and local buses. Essentially, it returns whatever data the DB Navigator app shows, including realtime delays and disruptions.
This API returns realtime data whenever its upstream, the API for DB's mobile app, provides it.
You can just use the API without authentication. There's a rate limit of 100 requests/minute set up.
This API has CORS enabled, so you can query it from any webpage.
This API sends
Cache-Control headers, allowing clients cache responses properly.